The aerobic composting of organic materials as a means of the disposal and/or recycling of such waste materials has gained increased acceptance in the recent past. Early work on composting technology was performed by Sir Albert Howard, a British agronomist, who developed the Indore Process through research performed in India. In 1972, the United States Department of Agriculture's Beltesville Agricultural Research Center began research for an improved method of windrow composting of sewage sludge. That research culminated in what is commonly known today as the "Beltesville process," as is set forth in a 1980 publication entitled "Manual for Composting Sewage Sludge by the Beltesville Aerated Pile Method." Additional such research on aerated static piles was performed by the New Jersey Agricultural Experiment Station at Cook College, Rutgers - the State University of New Jersey at the Camden County, New Jersey composting facility. The Rutgers research resulted in an improvement to the "Beltesville Process" primarily in the utilization of process control technology. This improvement has come to be known as the "Rutgers Method." Both the "Beltesville" and "Rutgers" static pile methods take place outdoors in compost piles of similar configuration. Two pile configurations were thus developed which employed individual static piles ("static" because each pile stands alone and is undisturbed or unagitated during a 21 day composting cycle), and the extended aeration process was used in which static piles from each day's production are stacked adjacent to one another to form one long, outdoor, continuous pile.
Along with the development of these open systems, enclosed systems were also being developed both in Europe and the United States. Europe's clusters of dense population centers dictated a need to recycle their organic waste in a manner which would have minimum impact upon local residents. The Europeans therefore developed large silo, tunnel and drum systems with internal process controls. In these systems, each of the composting units contains the entirety of the composting mass inputted over the composting cycle. These systems are represented by U.S. Pat. Nos. 4,236,910; 4,062,770; 4,184,269; 4,191,643; 4,161,426; 4,288,241; 4,132 638; and 4,255,389.
Such reactor-type systems were also developed in the United States, and these additional systems are represented by U.S. Pat. Nos. 4,139,640; 3,114,662; 4,138,333; 3,533,775; 3,556,420; 3,151,779; 3,438,710; 3,385,687; 3,291,491; 3,323,896; 3,963,470; 4,410,349; and 3,890,129.
These various open and closed composting systems currently utilize aerobic digestion to varying degrees in attempts to control the internal composting environment, as represented by the pH, moisture content, pile temperature, porosity, nutrient levels and oxygen levels for the compost pile. Of these two types of systems, the open pile systems require the minimum capital outlay. However, the actual design and construction of these open systems have resulted in unexpected operational and process performance problems. That is, because these piles are generally either triangular or trapezoidal in cross section, a temperature distribution occurs such that temperatures on the surface of the pile are significantly less (i.e., closer to ambient) than the temperatures at the center of the pile, thus yielding incomplete thermal pathogen destruction. Furthermore, insulation blankets of finished compost are sometimes utilized, but in extremely cold weather this does not negate these problems. In such cold weather, the outer surface also tends to freeze, and in addition leachate which is drawn into the blower casing may also freeze, thus causing blower failure. Furthermore, the exposure of all of the mechanical equipment to the open environment and freezing weather shortens the overall service life of this equipment. These open systems also create working conditions for plant operators which can become rather severe in both the winter and in the summer, when prolonged hot, dry periods can produce excessive dust. The mixing of such static pile systems is normally performed by mobile equipment, thus resulting in poor mix ratio control, as well as dense zones which can become anaerobic. Odor problems are common in open systems due to anaerobic pockets caused by poor mixing and ventilation patterns. Since these piles are exposed, dust problems can occur with resultant increased spore levels on the site. Furthermore, because these static piles normally undergo a 21 day aeration period, followed by a 30 to 45 day unaerated curing period, odor problems sometimes occur during the unaerated cure period. Also, in these open, forced aeration systems, it is generally not possible to collect exhaust gases for scrubbing of odor-causing compounds therefrom.
The various in-vessel systems have thus been designed to remedy many of these environmental problems associated with open static piles. In view of their high rates of digestion through very careful control of the internal environment, most of these systems are exemplified by an initial stabilization period within the vessel, followed by an aerated curing period. These systems also feature vessels which contain the entire volume of waste material production in one composting mass. That is, if the composting cycle is 14 days, then 14 days of waste material production are contained within the vessel. This arrangement thus results in expensive structures, as well as complex, expensive mechanical equipment, in order to accomplish the initial placement and extraction of the waste material. Control systems for these systems also tend to be complex, due to the amount of equipment which needs to be controlled. Furthermore, the extraction device is of great concern in such enclosed systems, since failure of same can result in prolonged vessel downtime, and providing for redundancy in the extraction device is technically difficult, expensive, and sometimes impossible. In any case, such extraction devices must also be serviced on a regular basis, and without redundancy the actual composting time per year could be reduced, since restarting a vessel (filling) could take as long as six weeks. Providing for redundancy of vessels, however, is an expensive solution to the problem. Further, the great depth of material results in expensive blowers, as well as a concern for internal air distribution patterns.
One attempt to solve these problems is shown in U.S. Pat. No. 4,288,241. In this system two open windrow-type piles are placed adjacent to each other, and air is drawn down through one of the piles by a blower and blown up through the second pile. Control is accomplished by a timer in this system, and the volume of air provided is intended to dry out the piles. Depleted moisture content is thus restored by adding wet sewage sludge during the process.
It is therefore an object of the present invention to overcome the deficiencies of this and other prior art systems by the provision of a modular composting system so as to accelerate that process.